This invention relates to lead-acid cells and batteries and, more particularly, to separators used in making such cells and batteries.
A wide variety of applications, often termed xe2x80x9cindustrial batteryxe2x80x9d applications, utilize conventional, flooded electrolyte lead-acid cells and batteries, or sealed lead-acid cells and batteries, often termed VRLA cells and batteries (xe2x80x9cvalve-regulated lead-acidxe2x80x9d). In stationary battery applications, the lead-acid cells and batteries provide stand-by power in the event of a power failure. For this type of application, such cells and batteries are maintained at a full state-of-charge and in a ready-to-use condition, typically by floating at a constant preset voltage. This voltage is in the range of 2.25 to 2.35 volts per cell to maintain the cells in a full state-of-charge while minimizing positive grid corrosion and electrolyte water loss. Stationary batteries are used for stand-by or operational power in a wide variety of applications, including, by way of illustration, telecommunications, utilities, for emergency lighting in commercial buildings, as stand-by power for cable television systems, and in interruptible power supplies for computer back-up power and the like.
Other applications in which lead-acid cells and batteries may be used involve a variety of motive power applications in which an array of cells or batteries provides the motive power for vehicles ranging from Class 1 to Class 3 trucks, various automated guided vehicles, mining vehicles and also railroad locomotives. The performance requirements for motive-power vehicles are quite different from the performance requirement for stationary power sources. In stationary power applications, the depth of discharge in service is relatively shallow, and the number of discharges encountered per unit time are fewer, as most batteries are in float service most of the time. In direct contrast, motive power applications require a relatively higher (i.e., deeper) depth of discharge to be achieved on a continuous cycling basis over a period of time, and while repeating such discharges on a continual basis. Indeed, a common requirement for Class 1-3 trucks is that, in an 8-hour shift, the cell or battery assembly must be capable of delivering an 80% depth of discharge and that performance is required for 300 cycles per year with a useful service life under those conditions of 4 or 5 years.
The widely varying requirements for these many applications have presented substantial problems and an extremely challenging environment in the design and the manufacture of lead-acid cells and batteries. This environment has resulted in, to a large extent, custom designs which satisfy particular applications.
As a consequence, lead-acid cell/battery manufacturers have had to develop families of cells and batteries in an attempt to satisfy the diverse electrical performance criteria. Such criteria vary widely, often requiring large cells connected in parallel, series, or both, to provide a satisfactory power/energy source.
The space requirements often are also quite constricted, with closely defined dimensional requirements. Many types of steel trays and the like are used to house the cells required.
To achieve the family of cells and batteries requires positive and negative grids of various sizes so that the capacity and other electrical performance requirements for an individual cell for a particular application can be satisfied. One approach utilized for VRLA cells has been to provide a series of grids having essentially constant width while varying the height of an individual grid and the number of plates used in a particular cell to achieve a variety of capacity levels or gradations and other electrical performance requirements. While an effective solution, this approach does create challenges that have to be addressed, as will be discussed hereinafter.
The internal configuration of as such VRLA cells can vary widely. In general, such cells are disclosed in U.S. Pat. No. 3,362,861 to McClelland et al. As is thus known, such cells utilize highly absorbent separators; and all of the necessary electrolyte is absorbed within the separators and plates. Such cells are normally sealed from the atmosphere by a valve designed to regulate the internal pressure within the cell so as to provide what is termed an effective xe2x80x9coxygen recombination cyclexe2x80x9d (hence, the use of the terms xe2x80x9csealedxe2x80x9d and xe2x80x9cvalve-regulatedxe2x80x9d as well as xe2x80x9crecombinantxe2x80x9d).
Recombinant battery separator materials (sometimes termed xe2x80x9cRBSMsxe2x80x9d) have traditionally comprised a highly absorbent glass microfiber mat. Separators of this type have adequate absorbency to hold the amount of electrolyte desired within the small pores and possess some vacancy of pores to allow the oxygen recombination cycle to proceed. A wide variety of suitable glass fiber mats are commercially available and are in use in VRLA cells and batteries. Glass microfiber is made from a borosilicate glass using a flame attenuation process that produces a microfiber with a diameter in the range from 0.25 to 4 xcexcm, and with a typical length of 0.8-1.5 xcexcm. These fibers have the consistency of cotton wool and are processed into a continuous porous sheet-form by a wet laying process on a paper-making machine. Typically, the glass microfiber mat will have a high porosity in the range of 85-95%, and this porosity contributes to the high electrolyte retention.
Despite the widespread use of such glass fiber mats, substantial efforts have been made to develop other recombinant battery separator materials, perceived to satisfy varying objectives. U.S. Pat. No. 4,908,282 to Badger summarizes many different prior art attempts to provide satisfactory separator materials for recombinant cells and batteries. Yet, Badger states that there has not previously been a suggestion of a separator which, when saturated with the electrolyte, leaves a residuum of unfilled voids through which a gas can transfer from one plate to another because the separator is not capable of holding an amount of electrolyte which is sufficient to fill all the voids (col. 2, 11. 20-26).
More particularly, Badger discloses a separator having, in general, two types of fibers. A first set of fibers imparts to the separator an absorbency greater than 90% relative to the electrolyte and a second set of fibers that have a different absorbency which is less than 80% relative to the electrolyte. The first and second fibers are disclosed as being present in proportions such that the absorbency of the overall separator is from 75-95%. Specifically, a separator is disclosed which is made of a mixture of two different grades of glass fibers, one grade of chopped glass strand and a certain grade of polyethylene fibers.
Another prior art attempt to provide a RBSM is U.S. Pat. No. 4,216,280 to Kono et al. The ""280 patent discloses separators which comprise glass fibers entangled in the shape of a sheet without the use of a binder and have a first and second portion of glass fibers. The first portion comprises glass fibers having a fiber diameter smaller than one micron; and a second portion uses glass fibers having a fiber diameter larger than 5 microns, as well as an average fiber length of at least 5 millimeters. Such separators are stated to have high electrolyte retention, good mechanical strength, and good shape recovery.
Yet another prior art attempt to provide RBSMs is U.S. Pat. No. 4,367,271 to Hasegawa et al. By way of background, the ""271 patent thus states that one prior proposal comprises a glass fiber mixed with a synthetic resin serving as a binding agent, while another type proposed involves mixing a glass fiber with a synthetic resin monofilament fiber. Hasegawa et al. state that such prior approaches are inadequate because these approaches suffer a remarkable decrease in liquid absorption and that the improvement in the mechanical strength is small. The ""271 patent is said to provide a separator which is high in liquid absorption, high in strength, and is easy to handle. Such separator materials, according to the ""271 patent, are produced by a process which uses glass fiber substantially 1 m2/g or more in specific area, mixed with about 10% or less, by weight, of fibril-formed synthetic fibers which have 350 cc or less in xe2x80x9cfreeness.xe2x80x9d
Still further, there are significant problems that arise during the service life of VRLA cells and batteries, particularly when the required service life is relatively long, that need to be overcome. First of all, as has been previously noted, one approach in this field provides a family of cells and batteries which utilizes grids of a constant width while the height of the grid is varied as well as a number of plates to provide the desired capacity and other electrical performance requirements. In cells of this type, and, indeed, in many cells that have a relatively large height, cell xe2x80x9cdry outxe2x80x9d due to rapid water loss from the electrolyte can become a prevalent problem, particularly, in industrial cells designed for a relatively long service life, e.g., 10 or 20 years or so. Such xe2x80x9cdry outxe2x80x9d can result from electrolyte loss, loss of intimate contact between the separators and the positive and negative plates or a combination of the two. Among the reasons for occurrence of xe2x80x9cdry outxe2x80x9d may include lack of appropriate resilience of the absorbent separator and/or formation of an electrolyte saturation differential from cell top to cell bottom. In addition, electrolyte stratification can cause a variety of problems such as sulfation of the negative plate and uneven usage of the plate from top to bottom.
Another problem, which perhaps may be related to cell dry out, is the loss of appropriate overall cell compression. More particularly, as is known, providing satisfactory electrical performance in VRLA cells requires intimate contact between the cell plates and the separators. Such contact is typically provided by compressing the separators by as much as 20% or more (based on their uncompressed thickness) in the cell so as to facilitate maintaining the necessary contact between separators and plates throughout cell life. What is not appreciated is the substantial changes that take place in the microglass fiber separator upon such compression. The porosity and surface area of an absorbent glass fiber mat may change dramatically with such levels of compression. Further, even with such compression, it is being discovered that absorbent glass fiber mats cannot sustain the needed compression throughout a long service life, resulting in premature cell or battery failure.
Another problem stemming from the use of absorbent glass fiber mats is their relatively low stiffness and mechanical strength. Such properties increase the susceptibility to separator damage during cell or battery assembly. Such damage could create internal shorts and lead to premature battery failure.
Despite these shortcomings and the considerable efforts in this field to achieve more desirable RBSM, absorbent glass fiber mats remain the material of choice for commercial VRLA cells and batteries. There certainly exists a clear need for improved separator materials, and for VRLA cells and batteries that can sustain the necessary electrical performance over the desired service life.
It is accordingly a principal object of the present invention to provide sealed lead-acid cells and batteries utilizing separators capable of enhancing the electrical performance over the service life of such cells and batteries.
A further and more specific object provides separators for large VRLA cells and batteries capable of maintaining higher resiliency and stable spring back characteristics in both dry- and acid-saturated conditions so as to maintain sustained pressure against the cell or battery plates in service.
Yet another specific object of this invention lies in the provision of such sealed cells and batteries having separators with improved thickness stability under varying magnitudes of separator compression and acid saturation.
Another object of the present invention is to provide separators for such sealed lead-acid cells and batteries exhibiting enhanced mechanical strength so as to facilitate cell and battery assembly.
A still further object lies in the provision of VRLA separators having good electrolyte wettability.
Other objects and advantages of the present invention will become apparent as the following description proceeds. While the present invention will be described primarily with respect to use in sealed lead-acid cells, it should be appreciated that the present invention can be advantageously used in any other application where separators of the type disclosed may find utility.
The present invention is, in general, predicated on the discovery that superior separators can be provided by utilizing a microfiber mat having at least 40% by weight modified polymeric microfibers and the balance, if any, comprising glass microfibers. It should be kept in mind that the same weight proportions, in a given weight of such modified polymeric microfibers, will yield a thicker separator than with equivalent weight-glass microfibers as the density of polymers is in the range of 0.95-1.2 g/cc, while that of glass is in the range of 2.0-2.5 g/cc. By suitably modifying such polymeric microfibers and by selection of appropriate characteristics, as will be discussed hereinafter, absorbent microfiber mats can be provided which, in comparison to glass mats, have improved mechanical properties for handling, assembly and service reliability and durability, which also achieve desirable thermal and electrochemical stability, oxidation resistance, superior resiliency and stable spring-back attributes among other characteristics.
Another aspect of the present invention utilizes a multilayered mat with at least one microfiber glass layer and one modified polymeric microfiber layer. In a preferred embodiment, the multilayered mat comprises two outer thin glass mats and a center polymeric layer.